Ok several things:
1) Route: I get the slightly more stable than "go where the wind blows me " routing but seriously your going to need to plan your trip routing a little bit tighter if for no other reason than security and peace of mind for the folks back home. If folks haven't heard from you in a bit truast me on this you do want your folks/U.S. Embassy folks, somebody to have at least an idea of where you are or are supposed to be.
2) Car Selection: Once you know your route, it will be easy to see just from the distances and the kind of stuff you like to do and see, where you are going to be spending most of your time. THEN pick the vehicle for that area, e.g, most of your time spent in Mexico with just a toe dipped into Guatemala, go with a vehicle that is easy to service in rural Mexico. If you plan to spend most of your time in Central America then fix up your 40, period. If the 40 is in decent shape you can spend 3k on the fix up and use the rest for your trip budget. By far I have yet to run into a small town in Central America where there wasn't a mechanic who knew a 40 like the back of his hand. 40, 80s and fourrunners are everywhere in Central America. BTW each of them comes in a diesel version here as well.

so cheap on the gas. If you can get lucky and catch a diesel one of any of these vehicles for sale in the U.S. ( they exist from time to time on
Ebay) so much the better. Whatever rig you pick make sure that it is near perfect running shape. I would use some of the extra cash ( see below) and replace all belts and hoses, etc with new and keep the old ones that are still in good shape as your spares. Basically everything as new as possible under the hood and car and keep the old peices as spares. Having your own spares as something busts in that little dusty town on a Sunday morning will greatly speed your departure. It is one thing to find the mechanic and convince him to get out of bed etc on a day off but it is another to convince him if you can show him that you already have all of the parts.
I know that people beat up 40s alot because of space issues. I can comfortably store all kinds of recovery gear and camping gear in mine and never even come close to breaking a sweat, i.e., plenty of space to spare ( if it is just you and you are not bring along all of your GF's stuff). If you are even slightly worried about space, go with the 4 runner and your troubles are completely over on that score.
3) Security: Low profile - I plan on making the same run but in reverse from Central America back up to the U.S. in my 40. I run the thing all over now and one of the things I like most is that I can roll though any small town here and it looks like every other car.
Gear: Travel light, keep it mostly inside and well tied down. Have great door locks and an alarm, everyone here does. Make sure that your windows are tinted as heavily as the law allows to avoid pryiing eyes and cut down on the heat. Keep two spare tires as the roads can get really crappy really fast and we do not want to mention the need for you to be super ready if you happen to roll through during the rainy season here so make sure that they are really good ATs and I think 4wd is a must here given the elevel of uncertainty in date and route planning here. Run two batteries with an isolator so you can give yourself a jump. Run either two tanks or a long range tank so if you need to you can avoid stopping in a sketchy place just because you are worried about gas. Remeber there is no AAA road side assistance here and the county mounties are not your friends in many cases.
4) Travel advice: Do what Charles, "Cruiser guy" says, period. He makes this run more often than anyone on here and he does it in cruisers, 55 and a 60, from Guatemala straight through and up to British Columbia at one point a couple times a year. If he syays stop at a certain place, gear up, wait and then make a run for the border at a certain time along a certain route, do it. He has in cruisers full of gear and family over and over again. If there is a customs beef that can happen he's been there and done that.
Sale of rig: Forget selling the rig, I have bought and sold a number of rigs here now and it is more than a casual traveler could pull off or bear. I t takes piles of paperwork and weeks to get right and that is if you spanish is fluent and you know your way around. Dance with the girl you bring to the dance, period.
5) Make the trip.
Good luck,
John